Wind turbines in HAWT design (horizontal axis) consist of four main parts as a structure, the base, the tower, the nacelle and the rotor with one or more blades.
The blades are mounted at fixed angularly spaced positions around the axis. The turbine includes a wind detection system which analyses the wind speed and direction repeatedly so as to repeatedly adjust the angle of the nacelle around a vertical axis of the tower, that is the angle of the rotor axis relative to the wind direction, and to adjust the angle of attack of the blades around the longitudinal axis of the blade relative to the wind speed.
Turbines if out of balance will cause significant vibration in the tower and can if sufficient rapidly deteriorate the structure or individual components to a position where it cannot be repaired or leave the turbine in an unsafe condition to operate in. Periodic checking of the state of balance must therefore be carried out.
Some manufacturers apply specific hardware and use methods such as theodolites or other template based methods to ensure ideal alignment between blades, without recording pictures but adjusting in real time.
Several attempts have been made to measure the angles remote from the ground with simple photograph based measurements but are not satisfactory. In most cases no suitable procedures were taken to verify measurements. In other cases vibration measurements have revealed results inconsistent with the measurements taken.
One attempt for the determination of a blade angle is described in patent application WO 2009/129617 (Jeffrey et al) published 29 Oct. 2009, the disclosure of which is incorporated herein by reference, or to which reference may be made for details not described herein.
When followed, the described methods in that patent application are unsatisfactory in that the measurements taken will include errors which up to now have not been determined so that this method has not lead to a satisfactory solution. Furthermore, the methods are of limited practical value and may provide a serious risk to the safe operation of a wind turbine when used as the base of further decision making.
In addition to analyzing the aerodynamic related moments and the mass moments, it is necessary to try to measure effectively the actual vibration that is occurring to determine whether any adjustments which are made are improving the total balance situation or are not. If the analysis of the aerodynamic related moments and the mass moments is based on faulty data, it is very possible that any adjustments made actually create a deteriorating situation so that the amount of vibration worsens.
Typically the evaluation of torsions vibration modes is made with two lateral sensors; but this is not suitable or reliable if the center of the torsional movement is not accurately determined. The arrangement of the nacelle mass, the hub and rotor mass as well as all other components mounted on the tower causes a situation where the center for torsional movement is not identical with the center of the tower so that this manner of measuring balance is inaccurate and can lead to faulty conclusions.
Furthermore it is known that rearrangements of heavier parts in the wind turbine as well as exchange of parts, for example the use of a lighter generator, can easily shift this center and lead to misleading readings on torsional vibration evaluation.
Up to now therefore balancing of turbines of this type has been ineffective and in some cases has lead to a situation where the results are confusing or even worsen the situation.
In view of the very high capital cost of this equipment, methods to accurately and inexpensively maintain the turbine in balance are very important.